It is known that roll film cartridges are constructed from metal or plastic. Plastic is a desirable material because of its light weight, strength and low cost. Standard methods of assembly exist for plastic components.
The two most common methods of assembling a roll film cartridge are presented in U.S. Pat. No. 3,128,058. The first method fastens two separately molded semicylindrical members together with a retaining clip or tape. A disadvantage of this process is the large number of individual components to be assembled. The second most common method of assembly has the two molded semicylindrical members joined by an integrated hinge along one longitudinal edge and a fastening means along the other. Generally, the roll film cartridge is formed by folding the semicylindrical members together before the hinge and members have substantially cooled.
For plastic hinges that must be capable of repeated flexing, numerous patents note that folding must occur before substantial cooling. This affects the flexibility of the manufacturing process, since it requires immediate action after injection molding. Plastic materials are comprised of long molecular chains and amorphous binders between these chains. When a plastic component is extruded the molecules tend to align themselves with the extrusion flow. U.S. Pat. No. 3,320,225 describes the process whereby the molecular chains of the thermoplastic may be reoriented to increase the flexure strength. When rolls apply a continuous pressure upon an extruded plastic hinge, the rolls reduce the plastic's cross section and consequently reorient the molecules. Location of the rolls relative to the extruder die orifice is critical because the working temperature range is narrow and temperature drops with distance from the die.
U.S. Pat. No. 3,301,430 describes the process of setting the extruded hinge into a fixed position as it cools, as well as flexing the hinge one to four times to reorient the materials before setting the hinge into a position for cooling. Again, application of this process to roll film cartridge assembly limits the flexibility for manufacturing. A need exists to manufacture the components separately and to assemble them after they have cooled.
When cool, the process of folding together the semicylindrical members creates stress cracks in the hinge, which may lead to a structural failure. Experimentation has shown that reheating the hinge anneals the plastic material and significantly increases its strength. Numerous means exist to reheat a substantially cooled roll film cartridge, but none localize the heat.
The heating must be localized at the hinge in order to prevent the possibility of structural changes in the semicylindrical members. The present invention uses directed hot air. Alternative means of localized heating include lasers and fiber optic light sources; however, the energy necessary from these systems to rapidly heat a hinge tends to either burn holes in the plastic or heat the hinge beyond the point of structural integrity. The heated air system, which combines convective and conductive heat trasfer, has the capability of penetrating the thickness of a plastic component without damaging the surface.
The present invention solves problems associated with the development of a sturdy, one piece, light proof cartridge. The present invention provides a means to fold a plastic component after it has been molded and substantially cooled. The present invention also provides a method to isolate the hinge and locally heat the hinge without affecting the structural integrity of the whole cartridge.